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This CL introduces AboutURLLoaderFactory::Create static method that
allows creating an AboutURLLoaderFactory that is owned by its
|receivers_| and will self-delete when the last receiver disconnects.

This CL removes the ability to directly construct and own a
std::unique_ptr<AboutURLLoaderFactory>, because this ability means that
the factory can be destructed while receivers bound via the Clone method
are still alive (see the associated bug).  In particular, this CL stops
exposing AboutURLLoaderFactory constructor as a public member, which
forces construction to go via the new Create static method.

This CL mostly just follows the pattern established earlier by
https://crrev.com/c/2337411.

Bug: 1106995
Change-Id: Id93cc5b5078019d95dd19988ffdffb8e40d7ac31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358053
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805584}